Improved powder formulations of high melting point thermoplastic aliphatic polyketones for selective laser sintering

Abstract

The invention relates to a powder composition for use in a selective laser sintering application, wherein said powder composition comprises aliphatic polyketones. The powder composition includes aliphatic polyketone with bulk melting point above 200°C. The powders obtention and characterization are described and show a plurality of particles. The formulation of powders based on non-spherical particles by dry blending with dedicated processing aids such as charged flow aid prior to selective laser sintering ensures improved processability and costs savings. The powder composition when used in selective laser sintering results in parts with more consistent results and improves process economics.

Description

IMPROVED POWDER FORMULATIONS OF HIGH MELTING POINT THERMOPLASTIC ALIPHATIC POLYKETONES FOR SELECTIVE LASER SINTERING

[0001] The invention relates to the field of powder compositions that can be used in additive manufacturing, particularly in selective laser sintering applications.

[0002] In particular, the invention relates to polymer containing powders used for the manufacturing of parts and structures, such as lightweight structures used in automotive, aircraft or space industry.

[0003] Besides, considerations on the preparation of such powders are also shared.

[0004] Selective Laser Sintering (SLS), also known as powder bed fusion, stands out as a distinctive 3D printing technology. It involves the creation of objects by selectively exposing successive layers of semi-crystalline thermoplastics or thermoplastic-elastomers powders to laser irradiation within a heated chamber, enabling their fusion.

[0005] Although SLS holds the potential to generate high-quality parts for practical use, its adoption in industry has been gradual. Polyamide 12 (PA 12) dominates approximately 80% of the SLS market, and alternatives, especially for high melting point thermoplastics (temperatures exceeding 200°C), remain scarce.

[0006] Noteworthy high melting point thermoplastics utilized in SLS include Polyetheretherketone (PEEK), Polyetherketoneketone (PEKK) and Polyaryletherketones (PAEKs), recognized as high-performance thermoplastics. However, their use is hindered by their relatively high cost.

[0007] It has to be noted that a variety of patents have been reported to ensure the printability of non-spherical powders of such high melting point polymers. Published patent document US 2021 / 0138689 A1 discloses a copolymer powder for use in SLS comprising a first fraction of polyetherketoneketone (PEKK) of non-spherical particles, and a second fraction comprising a plurality of carbon fibers.

[0008] Because they are inexpensive engineering plastics with good chemical resistance and physical characteristics, aliphatic polyketones are an emerging class of polymers. Typically, aliphatic polyketones are copolymers of carbon monoxide, ethylene and propylene (though vinylic, olefins or alkenes could be incorporated) as disclosed in U.S. Pat. No. 4,808,699. Those materials are of use in niche applications as replacement solutions for various polyamides (such as polyamide 6 (PA6)) or Polyoxymethylene (POM).

[0009] The patent application US20220363825A1 is the first one describing the use of powders of aliphatic polyketones composed of fine spheric particles for SLS. However, the materials exemplified have melting points lower than 200°C and the powders are based on particles of great sphericity.

[0010] However, in view of the powder composition disclosed by the document, there is still an opportunity for enhancing thermal properties of the aliphatic polyketones. This can be obtained by targeting materials with melting temperatures of the bulk higher than 200°C. Along those lines, there is a need for proper stabilization of such aliphatic polyketones with high melting point in the context of SLS printing, in order to ensure the absence of degradation by oxidation due to the combined effect of SLS chamber temperatures and laser irradiation.

[0011] Since, neat aliphatic polyketones that can be found commercially are supplied under the form of large powders, chips or granules, specific processes are required to turn them into fines powders which are appropriate for SLS (e.g., by cryogenic milling, precipitation or spray drying…).

[0012] However, when considering cryogenic milling, additional processing efforts are usually required to reach the particle sphericity required for SLS. Such efforts can be expensive. Therefore, is economically interesting to consider the formulation of powders based on non-perfectly spherical particle (e.g., with processing aids such as flow aids) to ensure their processability by SLS.

[0013] The present invention addresses the above-mentioned deficiencies and aims at providing a powder composition for SLS with enhanced mechanical properties, a high solvent resistance and abrasion resistance.

[0014] Overall, the present invention enables to manufacture products of higher quality, with a reliable and efficient powder composition.Solution

[0015] The above-stated problem is solved by a powder composition for use in a selective laser sintering application, wherein said powder composition comprises aliphatic polyketone.

[0016] According to a preferred embodiment, the aliphatic polyketone is contained in at least 20% by weight relative to the total powder composition.

[0017] According to a preferred embodiment, the aliphatic polyketone is aliphatic polyketone 330 or aliphatic polyketone 410 or aliphatic polyketone 930 or a mixture thereof.

[0018] According to a preferred embodiment, the aliphatic polyketone is a cryo-grinded powder comprising a particle size comprised in the range of 30μm to 100μm.

[0019] According to a preferred embodiment, the aliphatic polyketone comprises particles having a mean diameter D50 in the range of 40μm to 80μm.

[0020] According to a preferred embodiment, the aliphatic polyketone comprises a melting temperature comprised in the range of 200°C to 280°C.

[0021] The invention also relates to the use of the powder composition according to any of the above-mentioned embodiments for manufacturing an end product by a selective laser sintering process.

[0022] The invention also relates to a product obtained by the use the powder composition in accordance with the present invention.

[0023] The invention also relates to a method for manufacturing a three-dimensional product through selective laser sintering of a power composition, wherein said powder composition is according to any of the above-mentioned embodiments.

[0024] The invention can also relate to the following aspects:

[0025] Aspect {1}: machine for producing a three-dimensional product through selective laser sintering of a power composition, wherein said powder composition is according to any of the above-mentioned embodiments of the present invention.

[0026] Aspect {2}: machine according to aspect {1}, wherein said machine comprises a bed temperature able to rise up to 300°C.

[0027] Aspect {3}: method for preparing a powder composition for a selective laser sintering application, wherein said powder composition comprises aliphatic polyketone.

[0028] Aspect {4}: method according to aspect {3}, comprising the following steps; cryogenic milling, sieving, addition of flowing agent, and characterization of the powder composition.

[0029] Advantageously, the aliphatic polyketone (PK) containing powder composition comprises a high melting point (comprised between 220-260°C), and high mechanical properties, solvent resistance and abrasion resistance.

[0030] Compared to known polyamide (PA) containing powders for SLS, the powder composition on the invention enables to manufacture products with improved mechanical properties and enhanced chemical resistance.

[0031] The PK powder of the present invention is more cost effective compared to powders containing PEEK, PEKK or PAEK (high performance polymers).

[0032] The PK containing powder exhibits improved fusion and crystallization, and it possesses a good ability to absorb laser radiation without undergoing degradation.

[0033] Furthermore, PK is a thermoplastic that is easily recyclable.Brief description of the drawings

[0034] shows a schematic representation of an aliphatic polyketone structure of the powder composition according to the invention;

[0035] shows a schematic representation of another aliphatic polyketone structure of the powder composition according to the invention, which is more generic;

[0036] shows a comparison of the monomer composition of each aliphatic polyketone considered in this invention. Measurement have been obtained by proton NMR.

[0037] is a comparative graph derived from thermal gravimetric analysis of aliphatic polyketone under air and under nitrogen;

[0038] is a comparative graph obtained from thermal gravimetric analysis conducted under air conditions between aliphatic polyketone of the invention, and other polymer powders from prior art;

[0039] shows a comparison of thermal properties of the bulk of aliphatic polyketone powders of the invention and the references: polyamide 6 / 12 powders;

[0040] shows the comparison of the melting point obtained during the first and second heating scans by differential scanning calorimetry for each aliphatic polyketone powders considered in this invention.

[0041] is a differential scanning calorimetry plot of the bulk of a reference material : PA12;

[0042] is a differential scanning calorimetry plot of the bulk of a reference material : PA6;

[0043] is a differential scanning calorimetry plot of a polyketone (PK410) powder of the composition of this invention;

[0044] is a differential scanning calorimetry plot of a polyketone (PK930) powder of the composition of this invention;

[0045] is a differential scanning calorimetry plot of a polyketone (330) powder of the composition of this invention;

[0046] is a differential scanning calorimetry plot of a polyketone (PK) powder of the composition of this invention;

[0047] is a differential scanning calorimetry plot of all polyketone powders considered in this invention. A comparison of the first and second heating scans is made for each material;

[0048] illustrates the main processes that can be used for preparation of the polymer powder composition. According to the present invention only the most economic approach, cryo-milling, has been considered;

[0049] shows SEM micrographs of particles contained in powder compositions from each of polyamide 12, polyamide 6 and aliphatic polyketone of the invention, compared before and after exposure to heat;

[0050] shows SEM micrographs of particles contained in powder compositions from each of polyamide 12, polyamide 6 and aliphatic polyketone of the invention;

[0051] shows a comparison between properties of polyamide 12 powder and aliphatic polyketone powder according to the invention (PK);

[0052] is a schematic representation of a method for measuring powder flowability;

[0053] is a schematic representation of a method for measuring powder compressibility characteristics;

[0054] shows a comparison between compressibility characteristics of polyamide 12 powder and aliphatic polyketone powder according to the invention PK / NP(-);

[0055] is a schematic representation of a method for measuring powder charge density;

[0056] is a graph showing different measured charge densities of the aliphatic polyketone powder of the invention in comparison with polyamide powders from prior art;

[0057] is a graph showing measured charge densities of different types of aliphatic polyketone powders according to the invention, some of which are charged with flowing agent additives;

[0058] is a schematic representation of a method for measuring dynamic cohesion;

[0059] is a graph comparing measured dynamic cohesion with the method illustrated in, of polyamide and the aliphatic polyketone powder of the invention;

[0060] is a schematic representation of the main parameters to be considered when optimizing the SLS processing window of a new polymeric powder;

[0061] is the equation used to estimate the density of energy, which is known to be a useful tool to delimit the SLS processing window of a polymeric powder;

[0062] shows a comparison of SLS parameters used to print the selected aliphatic polyketone powder formulation PK / NP(-)of the invention and polyamide powders commercial references (prior art);

[0063] represent the impact of proper formulation of aliphatic polyketone powder on the main defects which can be observed on SLS printing bed and printed parts

[0064] is a graph depicting a comparison of the maximum yield strain test results conducted on known polyamide powders and the aliphatic polyketone powder PK / NP(-)of the invention;

[0065] discloses a graph depicting a comparison of the cyclic tensile test results conducted on known polyamide powders and the aliphatic polyketone powder PK / NP(-)of the invention.Detailed description of the drawings

[0066] Advantageously, the powder composition, as per the invention, comprises a thermoplastic polymer, corresponding to aliphatic polyketone (PK).

[0067] The PK of the powder of the invention comprises a structure according to the one illustrated in, or to the one of. The former having a melting point of around 220°C, and the latter having a melting point of around 260°C. PK is however not limited the structures discloses in figures 2 and 3, and can have different melting points from 200 to 280°C.

[0068] Preferably, PK can be procured as flakes or granules from Hyosung Chem.©, a company that produces PK under the brand name POKETONE™, or from AKRO-PLASTIC© under the brand name AKROTEK® PK-XM natural (5476), which is a compound based on Polyketone 330 F.

[0069] Aliphatic polyketone (PK) in the powder composition of the invention is preferably selected from the list: PK, PK330, PK410 or PK930, or a mixture thereof. Other types or grades of PK can also be used, such as PK630, PK710, or PK730.

[0070] PK from said list corresponds preferably to AKROTEK® PK-XM natural (5476), while PK330, PK410 and PK930 can be produced from Hyosung Chem.©. Said PK is also referred to as “compounded PK” in the present description.

[0071] Advantageously, PK copolymers have various monomer ratios and molecular weights leading to a variety of melting points and melt flow. For instance, PK410 has a melting point of 200°C and a melting flow of 30g / 10min, while PK330 and PK930 comprise a melting point of 220°C a melting flow of 60 and 200g / 10min, respectively.

[0072] PK is preferably contained in at least 20% by weight relative to the total powder composition. More preferably, PK is contained in at least 50% by weight relative to the total powder composition. Such amount allows the mixture of PK with other different components (for instance polyamide), or a mixture of different grades of PK.

[0073] Even more preferably, PK is contained in the range of 80wt% to 100wt% in the total powder composition of the invention.

[0074] Raw PK particles preferably undergo cryogenic milling using a cryo-grinder and sieving technique, in which needle discs of the pin grinder rotate at high speed, enabling low operating cost and avoiding spherical particles. Small particles of PK under 10μm were removed. Obtaining non-spherical particles shape having an average particle size comprised in the range of 10μm to 300μm.

[0075] is a comparative graph derived from thermal gravimetric analysis (TGA) of PK under air and under nitrogen, at a temperature variation rate of 10 Kelin per minutes.

[0076] It can been seen that PK is stable thermally. At high temperatures (above 500°C) a sensitivity to oxidation can be observed. Therefore nitrogen gaz is preferred during the SLS printing process, along the addition of specific stabilizers (melt compounding) to PK particles.

[0077] is a comparative graph obtained from thermal gravimetric analysis conducted under air conditions between PK of the invention, and other polymer powders from prior art, including PA6NC, PA6 and PA12.

[0078] It can be observed that compounded PK is more stable than all other PKs (PK330, PK410 or PK930).

[0079] shows a comparison of thermo-mechanical properties between the PK powders of the invention and PA12 and PA6 powders.

[0080] In the table shown in, Tg stands for the temperature of glass transition, Tc represents the crystallization temperature, and Tm corresponds to the melting temperature.

[0081] A larger temperature window (Tm-Tc) is generally beneficial for improved control and quality in the SLS printing process. Advantageously, the window of PK is quite similar to the one of PA6.

[0082] to 13 are graphs illustrating the thermal properties of all PK powders versus those of PA 12 and PA6 references, showing the high melting point of those materials and summarized in the table of.

[0083] In addition theand the table insummarize the effect of thermal pre-treatment on the melting point of all high melting point aliphatic polyketones. It shows that it is possible to remove the first melting peak of most of those materials by a proper thermal pretreatment, thus increasing the SLS thermal processing window.

[0084] Here the graph further illustrates the evolution of the specific heat flow (W / g) measured with differential scanning calorimetry (DSC).

[0085] illustrates the main processes that can be used for preparation of the polymer powder composition. According to the present invention only the most economic approach, cryo-milling, has been considered;

[0086] Preferably, the PK containing powder composition of the invention underwent or comprises at least one of the following: commercial powders blending, powder surface modifications, functional filler blending, additivation, stabilization, cryogenic milling and blending of polymers, blends or synthesis of polymers, blends of polymers / fillers, chemical functionalization, etc.

[0087] shows optical-microscope micrographs (at a scale of 100 µm) of particles contained in powder compositions from each of PA12, PA6 and PK of the invention, compared before and after exposure to heat on a heating plate.

[0088] As the temperature of PK increases, one can observe a phenomenon of particle fusion among themselves, quite similar to PA6. Advantageously, the fusion and combination of particles of PK lead to the formation of a cohesive and integrated printed structure with SLS.

[0089] shows SEM micrographs of particles contained in powder compositions from each of polyamide 12, polyamide 6 and aliphatic polyketone of the invention. While the size of the particle are quite similar, their sphericity is significantly different. Indeed the aliphatic polyketones have been cryo-grinded without any further physical post-treatment on this picture.

[0090] Both PA12 and PA6 SEM micrographs are observed at a scale of 100μm, while PK is observed at 200μm.

[0091] It can be seen that PA12 and PA6 particles are spherical, such a particle shape (sphericity) is observed after cryogenic milling (similarly to the cryogenic milling of PK particles) and may be obtained due to an applied ageing process.

[0092] While PK particles are advantageously not spherical at all, and having a mean particle size of around 75µm. Such a flake-like shape of PK particles enhances mechanical properties of the 3D manufactured end product with the SLS machine of.

[0093] Preferably, PK particles of the powder of the invention comprise a mean diameter D50 in the range of 40μm to 100μm.

[0094] shows a comparison between properties of PA12 powder and PK powder according to the invention.

[0095] Despite differences between PA12 and PK (density and Hausner coeff.), D50 values of both powders are really close.

[0096] is a schematic representation of a method for measuring powder flowability.

[0097] Preferably, said method uses a GranuHeap™ machine 19 which granular material heap analyzer, enabling automated repose angle α and cohesive index measurements.

[0098] The machine 19 comprises a initialization tube 19.1 configured to drop powder on a cylindrical support 19.2, which enables to obtain a conical pile of powder 19.3 to obtain the heap angle α (which depends on the grains shape). The smaller the angle, the better the fluidity of the powder.

[0099] The results of PK powder of the invention closely resemble those of PA12 and PA6, with the angle α being approximately similar, meaning that powder flowability of PK is quite similar to known polyamide powders.

[0100] is a schematic representation the principle of a method for measuring powder compressibility characteristics;

[0101] Preferably this method is called Granupack™ in which the powder compressibility is obtained after 500 taps, and is call the Carr index.

[0102] shows a comparison between compressibility characteristics of polyamide 12 and 6 powders and aliphatic polyketone powder according to the invention PK / NP(-). It is demonstrated that values are in the same range of the two commercial references PA12 and PA6.

[0103] is a schematic representation of a method for measuring charge density of the PK powder.

[0104] Preferably, said method uses a GranuCharge™ machine 20 which is a granular material charge analyzer, enabling electrostatic charges measurement of powder flowing on a surface. Said machine 20 comprises a vibration feeder (not illustrated) of the powder 20.1, and two tubes 20.2 and 20.3 (SS316L Pipes) dropping powder particles in a faraday cup 20.4. The machine 20 enabled to provide comparative measurements shown in.

[0105] is a graph showing different measured charge densities of the PK powder of the invention in comparison with polyamide powders from prior art.

[0106] One can observe that measured charge densities of all PK powders Δq are significantly higher than those of PA12 and PA6.

[0107] Preferably, the PK containing powder composition of the present invention undergoes a process where electrostatic charges are dispersed using charged nanoparticles (NP). More preferably, using fumed aluminium oxide (Al2O3) nanoparticles, comprising NP(+): Aeroxide® Alu C (Evonik) ~ positively charged used for electrostatic charge control, and / or NP(-): Aeroxide® hydrophobic Alu C 805 (Evonik) ~ negatively charged used for to control electrical resistivity.

[0108] Flowing agents (charged NP) were advantageously introduced at 0.5wt% relative to the total powder composition to enable electrostatic charges dispersion in the PK powder.

[0109] is a graph showing measured charge densities (measured at 25°C) of different types of aliphatic polyketone powders according to the invention, some of which are charged with the flowing agent additives NP(+)and NP(-).

[0110] The flowing agent NP(+)and NP(-)introduced respectively at 0.5wt%, do improve powder charge density build-up.

[0111] It can be seen that PK / NP(-)has a Δq equivalent to that of PA12 (and < PA6).

[0112] The dotted arrows illustrate a reduction in charge density over the precedent results shown in the graph of(prior to adding flowing agents).

[0113] is a schematic representation of a method for measuring dynamic cohesion of powders

[0114] Preferably, said method uses a GranuDrum™ machine 30 which is a powder tester for cohesion, that comprises a rotating drum 30.1 filled with powder 30.2.

[0115] On the right side of, we can see a sketch 30.3 of the dynamic cohesion measurement. In fact, dynamic cohesion is proportional to the flowing angle β. A decrease in said angle corresponds to a decrease in cohesion, and conversely, an increase in the angle corresponds to an increase in cohesion.

[0116] Cohesive forces depend on at least one of: Van der Waals force, electrostatic force, inter grains friction, and grains shape.

[0117] Comparative measures of dynamic cohesion have been performed by the inventors with at 25°C and a drum 30.3 rotation speed ranging from 5 to 60rpm.

[0118] It has been found that flowing agent NP(-)introduced at 0.5wt%, does improve PK powder dynamic cohesion to reach the same range of values than PA12, particularly starting from 30rpm, as it can be seen in the graph of.

[0119] is a graph comparing measured dynamic cohesion with the method illustrated in, of polyamide 12 and the aliphatic polyketone powder of the invention with and without charged flowing agent NP. It shows that forthis specific composition, the improvement is better with negative NP than with positive one.

[0120] is a schematic representation of the main parameters to be considered when optimizing the SLS processing window of a new polymeric powder;

[0121] is the equation used to estimate the density of energy, which is known to be a useful tool to delimit the SLS processing window of a polymeric powder;

[0122] shows a comparison of SLS parameters used to print the selected aliphatic polyketone powder formulation PK / NP(-)of the invention and polyamide powders commercial references (prior art);

[0123] ED stands for “energy density” which is measured by the following relation: (laser power / (scanning speed*hatch spacing)). The distance covered by each scanning interval is of 0.1mm. Laser speed in 0.06mm Pt / s.

[0124] PK processing window could be defined thanks only to: visual observations and / or ED calculation. It has been observed that PK processing window is thin (similarly to the one of PA6).

[0125] represent the impact of proper formulation of aliphatic polyketone powder on the main defects which can be observed on SLS printing bed and printed parts. It is obvious that the combination of static electricity and flow improvement improve the quality of both the printing bed and the printed specimens.

[0126] is a graph depicting a comparison of the maximum yield strain test results conducted on known polyamide powders (PA12, PA6) and the PK powder of the invention.

[0127] The tensile test respects the standard: ASTM D638-14 / S8, performed at 1mm / min.

[0128] Advantageously, PK gives a better max Yield than PA12 and PA6. Which improves printing conditions with PK compared to polyamide.

[0129] discloses a graph depicting a comparison of the cyclic tensile test results conducted on known polyamide powders and the PK powder of the invention.

[0130] The cyclic tensile test respects the standard: DIN 53534 / S2, performed at 10mm / min, with 10 cycles at 10%, 30% and 50% max elongation.

[0131] Results show that the tensile stress of PK is superior to PA6. The Volume strain accumulation increases with the strain for all materials.

Claims

A powder composition for use in a selective laser sintering application, characterized in that said powder composition comprises an aliphatic polyketone of a melting point higher than 200°C, wherein the aliphatic polyketone is selected from aliphatic polyketone 330, aliphatic polyketone 410, aliphatic polyketone 930, or a mixture thereof.Powder composition according to claim 1, wherein the high meting point aliphatic polyketone is contained in at least 20% by weight relative to the total powder composition.Powder composition according to any of claims 1 or 2, wherein the high melting point aliphatic polyketone is based on ethylene and propylene monomers, with an ethylene / propylene monomer ratio, as measured by NMR, ranging from 85 / 12 to 100 / 0.Powder composition according to any of claims 1 to 3, wherein the high meting point aliphatic polyketone is a cryo-grinded powder comprising a particle size comprised in the range of 10μm to 300μm, measured by laser diffraction.Powder composition according to any of claims 1 to 4, wherein the high melting point aliphatic polyketone comprises particles having a mean diameter D50 in the range of 40μm to 80μm, measured by laser diffraction.Powder composition according to any of claims 1 to 5, wherein the high melting point aliphatic polyketone comprises a melting temperature comprised in the range of 200°C to 260°C, determined by differential scanning calorimetry (DSC).Powder composition according to any of claims 1 to 6, wherein flowing agents are introduced in the range of 0.1 to 5 wt%, preferably 0.5wt% relative to the total powder composition.Powder composition according to claim 7, wherein the flowing agents comprise charged NP.Use of a powder composition comprising aliphatic polyketone of a melting point higher than 200°C for manufacturing an end product by a selective laser sintering process.Use according to claim 9, wherein the powder composition is according to any of claims 1 to 8.Product obtained by the use the powder composition in accordance with any of claims 9 and 10.Method for manufacturing a three-dimensional product through selective laser sintering of a power composition, characterized in that said powder composition comprises aliphatic polyketone of a melting point higher than 200°C.Method according to claim 12, wherein the powder composition is according to any of claims 1 to 8.

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